The term labyrinth seal describes a packing, or seal, between a stationary portion, referred to as the land, or stator, and a rotating portion, or rotor, of a machine. The labryinth seal includes a series of regularly spaced teeth that extend around the rotor or stator. The teeth form a throttling clearance from the opposite wall. A gas or steam passes through the throttling clearance in a jet stream which impinges upon the following tooth so that turbulence is created in the gas or steam and kinetic energy is thus greatly reduced. Basic types of labyrinth packings, or seals, for steam turbines are summarized in a publication entitled "The Leakage of Steam Through Labyrinth Seals" by Adolf Egli in Transactions of The American Society of Mechanical Engineers, Fuel and Steam Power, vol. 57., pages 115-121, 1935. FIG. 8 therein schematically illustrates two types of straight-through-type labyrinth seals. FIG. 9 schematically illustrates two types of staggered-type labyrinth seals. A staggered seal is a type of seal having a series of cavities defined by an element of the labyrinth seal. The first type of staggered seal has upstream and downstream sides having the same diameters; the second type of staggered seal has upstream and downstream sides relative to the direction of leakage flow having different diameters. FIG. 1 of the present application reproduces the first type of the staggered-type labyrinth seal of FIG. 9 of the Egli publication.
New types of labyrinth seals are described in a study entitled "Aerodynamic Performance of Conventional and Advanced Design Labyrinth Seals With Solid-Smooth, Abradable, and Honeycomb Lands", by H. L. Stocker, D. M. Cox, and G. F. Holle, prepared for National Aeronautics and Space Administration, NASA Lewis Research Center, Contract NAS 3-20056, Nov., 1977. FIG. 72 on page 108 of the study schematically illustrates a stepped large-to-small diameter optimized advanced labyrinth seal configuration which includes four rotor-connected teeth slanted against the direction of flow of a gas or steam. FIG. 81 on page 117 of the study schematically illustrates a stepped small-to-large diameter optimized advanced labyrinth seal configuration which includes a notched land, or stator, and four rotor-connected teeth slanted in the direction of flow of a gas or steam. Prior art FIGS. 2 and 3 of the present application reproduces FIGS. 72 and 81, respectively of the NASA study. Prior art FIGS. 2 and 3 are essentially the same stepped labyrinth seal with the flow direction of the leakage fluid reversed and with the direction of the slant of the teeth likewise reversed. The experimental study showed that approximately a 20 percent improvement was obtained with the new type of slanted teeth seal. The improvement was in part dependent upon the small clearance between the rotor and the stator. Because of the proximity between the slanted teeth and the lip of the notches, application of the described new design would subject the seal to axial rubs during startup of a gas or steam turbine because of large and differentiated axial movements that occur during that time. This is especially true for a gas turbine, which can reach full load in about ten minutes. It is noted that the prevention of such axial rubs caused by the presence of slant teeth can be accomplished by increasing the axial length of the seal, which is an undesirable aspect of the new slanted teeth labyrinth seals.
One problem that exists relating to labyrinth seals and gas turbines is as follows. Cooling air extracted from the compressor is required in a modern gas turbine to maintain the turbine components within acceptable temperature limits in order that they maintain their mechanical integrity. Such extracted cooling air is supplied to the downstream, or rearward, side of the wheels. A small portion of this cooling air is intentionally leaked through a space between the stationary downstream nozzle and the rotating downstream spacer to the forward side of the following wheel. The intentional leakage flow through this space is restricted by a labyrinth seal formed by the downstream spacer and a diaphragm connected to the downstream nozzle. In the present art of the labyrinth seal, the amount of intentionally leaked cooling air exceeds that needed to cool the forward side of the following wheel. Because of this, excessive cooling flow is extracted from the compressor with the result that the output of the machine as a whole is reduced.